1. Field of Invention
The current disclosure is directed to apparatus and methods of printing features on substrates, such as conductive features on semiconductor substrates, and more specifically, to the printing of metallization contact lines on the surfaces of photovoltaic devices, as well as to methods for forming stencils for performing semiconductor metallization processes.
2. Discussion of Related Art
In solar cells, front side metallization allows charge built up on a front surface of a solar cell during use to be transported away from the cell and used to provide electrical power. This front side metallization is deposited on the surface of a solar cell as part of the manufacturing of the solar cell. One methodology for deposition of the front side metallization may employ screen printing techniques to deposit conductive metallization ink onto the surface of the solar cell. Specifically, in certain methods, a conductive metallization ink may be deposited on a substrate through a screen having a pattern defined by openings provided in the screen. A screen utilized for this application may consist, for example, of a 400 mesh polyester or stainless steel carrier with an emulsion image for printing.
There are several well-known difficulties associated with printing conductive metallization ink with a screen printer onto an electronic substrate or solar cell. For example, the metallization ink flow may be obstructed by the carrier mesh. Another difficulty may be the loss of line definition and resultant ragged edges caused by the mesh, which deteriorates over time because of the wear of the emulsion screen. Yet another difficulty is the screen printer's inability to print small line widths due to bleeding of the metallization ink at the contact interface between the screen and the substrate. A further difficulty is the screen printer's inability to print high aspect ratio features because of bleeding of the metallization ink at the contact interface between the screen and the substrate or because of slumping of the ink, leading to widened and topped-off lines.
One possible solution to most of these difficulties might be to increase the viscosity of the metallization ink. However, changing the rheology of the metallization ink to a more viscous type does not adequately solve these problems as the screen may clog very easily when a more viscous ink is used.
Stencils are used in the electronics assembly industry for printing solder paste on electronic circuit boards (also known as printed circuit boards (PCBs) or printed wiring boards (PWBs)) in order to facilitate the attachment of electronic components via a process known as surface mount technology. (See, for example, U.S. Pat. No. 6,988,652, “SOLDER PRINTING USING A STENCIL HAVING A REVERSE-TAPERED APERTURE” to Fleck et al., describing a stencil used for printing solder paste on a contact pad of a printed wiring board.) However, most stencils, such as those used in the electronics assembly industry, typically exhibit significant bleeding and subsequent line broadening and irregularity when used to deposit metallization ink, or other less viscous materials, on semiconductor substrates, making them generally unsuitable for such an application.
The production of flat or three dimensional metal parts, such as stencils, by electroforming on mandrels, is known in the industry. For many two dimensional parts, photoresist technology may be used to prepare the mandrel, typically consisting of a conductive base plate with an image formed by photoresist adhered to it. In known methods for imaging, a mask, such as a plotted silverfilm mask, may be brought in intimate contact with the photosensitive plate to ensure a 1:1 reproduction of the image. When imaging thick photoresist layers, e.g., 100 micrometers, through a mask by a collimated light source, such as a Tamarack 161 collimated exposure system, the resist structure typically shows a tapered form with an angle of about 6 degrees, with the top of the image being smaller than the bottom, thereby limiting the resolution of narrow lines.
Using special, and somewhat complicated techniques, some modeling of a shape of an aperture that may be formed by photoresist technology may be achieved. For instance, in patent application DE 2828625 by Siemens, the following technique is discussed: rotating a light source, mounted under an angle, about an axis perpendicular to a photoresist covered plate. This rotation produces a controllable, circular, symmetric angle of taper in the exposed photoresist.